Silk is a fiber harvested from the cocoon of the silkworm. It makes an incredibly soft, smooth, and shiny fabric, and is highly prized for all sorts of luxury apparel and bedding. Silk has been in use in China since at least 3000 BC, and it was likely being harvested much earlier than that.
Robes are one of the more popular uses of silk in clothing. They are usually intended for bedroom wear, and they are ideal for this setting because of the sensual nature often associated with both the look and feel of the fabric. Often silk robes are made to go with a specific set of lingerie, matching in color and general appearance.
When buying silk robes, there are only two real factors to keep in mind. Most important is to ensure that the robes you are buying are in fact made from 100% silk, rather than a silk blend or "synthetic silk", usually nylon or polyester. Occasionally, silk robes will be composed of silk blended with acetate, a wood fiber similar to silk, but more fragile and susceptible to damage when being washed. Some silk robes use 100% silk, but then add an additional fabric as lining -- so long as the second fabric is soft enough to complement the silk, this shouldn't be a problem. For example, many high-end silk robes have cashmere linings to protect the silk from damage and to offer a second sensual fabric.
Don't be confused by the term satin if it is used to describe your silk robes. Satin can be made of a number of fibers, one of which is silk. As long as the satin is somewhere described as being silk satin, as opposed to nylon or acetate, what you are purchasing is still a silk robe. Most people prefer woven satin for silk robes, in contrast to cheaper knit satin -- the difference is one of texture, with woven silk satin being much softer to the touch than the rough surface of knit satin.
The only other thing you need to concern yourself with when buying silk robes is whether you enjoy the style and color. Silk robes come in a wide range of styles, some designed specifically for women and some for men. Some are meant to be showy and act as a supplement to lingerie, while others are meant for relaxation and comfort. Most are clear about length and cut, and as long as you pay attention to what you're purchasing, you should be pleased with the result.
If properly treated, and not subjected to much direct sunlight, silk robes can last for years, particularly if buttressed with a fiber like cashmere or angora. Though many people associate high costs with silk robes, the truth is they can be pleasantly affordable. At the low end, silk robes can be found for under US$50, while high-quality, well-made robes may be found for as little as US$150.